The short answer: Prism glasses carry a wedge-shaped correction that bends light before it reaches your eye, so both eyes point at the same image. Opticians prescribe prism for eye misalignment that causes double vision, eye strain, or headaches. It shows on your prescription as a prism amount and a base direction, separate from your regular power.

How prism correction works

A prism lens is thicker on one edge than the other, a gentle wedge. Light bends toward the thick edge, which shifts the image your eye receives. If one eye sits slightly off, the prism moves the image to meet it, so your brain fuses the two views into one. The rest of the lens still carries your normal distance or reading power.

Who needs prism

Prism treats binocular vision problems, where the eyes do not aim together. Common signs are double vision, words that swim or overlap when you read, eye strain, headaches, and a pulling feeling around the eyes. These can come from a muscle imbalance, a long-standing eye turn, or a neurological cause, so a proper exam matters. If you notice these symptoms, book an eye exam rather than guessing.

How it appears on your prescription

Prism adds two entries per eye: an amount, measured in prism diopters, and a base direction, which is where the thick edge points. Base directions are up, down, in toward the nose, or out toward the ear. You might see something like 2 base out. It sits next to your sphere and cylinder. Our guide on reading your prescription covers the rest of the line.

What to expect

Prism lenses can be slightly thicker on one side, and stronger amounts add weight, where high-index materials help. Most people adapt within days as the double vision settles. Prism is a medical correction, not a style choice, so it comes only from an eye exam. Do not order it from an old or guessed number.

From our test bench. Prism is the one correction we always send to an optometrist rather than treat as a frame question. What we can say from fitting them: pair prism with a sturdy, well-centered frame, because the correction depends on the lens sitting exactly in front of the pupil. A frame that slides undoes the alignment the prism is there to fix.

FAQ

What do prism glasses do?

They bend light with a wedge-shaped lens so both eyes point at the same image. This corrects eye misalignment that causes double vision, eye strain, or headaches.

How do I know if I need prism lenses?

Signs include double vision, words that swim or overlap, eye strain, and headaches. Only an eye exam can confirm it and measure the amount, so book one if you notice these.

Are prism lenses thicker?

They can be, on the side with the thick edge, and stronger prism adds weight. High-index materials keep them slimmer and lighter.

Part of Sunglasses Lenses Guide · Related: how to read your prescription · Browse eyeglasses · Last updated: August 5, 2026